Linear velocity sensors (or simply velocity sensors) may play a very important role in the control of any mechanical system motion, or vibration that may be encountered by the mechanical system. For example, in the automotive industry, luxury vehicles are expected to provide “premium” performance, such as a smoother, or quieter ride, under a wide range of varying operational conditions. It is known that adaptive or proactive control may be provided to mechanically tune the suspension system, such as for the vehicle chassis, and/or the engine mounts, to accomplish this premium performance. Either an adaptive or a proactive vibration control system needs to sense system motion, such as relative displacements and relative velocities in order to select an appropriate vibration control strategy. Typically, a “linear relative velocity” sensor is used to provide the mechanical system velocity information for these types of control systems.
There are presently a number of velocity sensing techniques that can provide the relative velocity information. They could be broadly divided into three categories:
1) mathematically differentiating a position signal;
2) directly measuring velocity; or
3) mathematically integrating an acceleration signal.
The first technique is somewhat limited since the derivative of the position signal generally introduces an unacceptable high level of noise to the resulting signal, and, consequently, lacks sufficient accuracy for meeting high-performance system requirements. The third technique has generally very good noise immunity because of the filtering-effects provided by the integration. However, for this sensing technique to work, the initial condition(s) of the system are required. This would result in additional complexity that incrementally adds to the cost of the sensor. Further, this technique may undesirably introduce relatively large phase delays. The second technique for sensing velocity, i.e., a direct measurement method, generally provides a versatile design choice to the designer because it avoids the issues concomitant with the first and third sensing techniques.
One common way to measure relative velocity is to use a sensor made up of a coil and a magnet mounted on a movable plunger. The coil is typically made with a uniformly distributed winding. That is, the number of turns is constant along the longitudinal axis of the spool on which the coil is wound. Theoretically, the coil voltage output signal should be proportional to the rate of change (e.g., speed) of the flux developed within the coil. Although this type of design has proven to be useful to generate velocity information, there are some issues that have yet to be addressed.
One issue that needs to be addressed is the fact that the output signal indicative of velocity information is undesirably dependent upon the plunger position. For example, as the plunger moves to an extended position from a retracted position over an exemplary practical range of plunger travel (represented in FIG. 1 by a line 10 with twin-headed arrows) the coil output signal increases, notwithstanding of a constant positive velocity, e.g., velocity V1. Conversely, over the same range of plunger travel, the coil output would decrease in the presence of a constant negative velocity −V1. That is, the output level of the signal indicative of velocity varies not just as a function of the actual velocity sensed by the coil but also as a function of plunger position. Thus, even though the velocity being sensed is in fact constant, the output signal from the coil varies depending on the travel position of the plunger.
Another factor for reducing the sensor cost is the ability to package relatively small components within the sensor housing, e.g., the magnet and coil. Unfortunately, the size of the magnet used in various known configurations, may result in a relatively bulky sensor, which is generally undesirable for applications where spacing may be scarce.
In view of the foregoing considerations it would be desirable to provide a velocity sensor with an improved linear output range. That is, a sensor having an output signal that varies essentially as function of the actual velocity sensed by the sensor and exhibits reduced sensitivity to plunger position. It would be further desirable to provide in a smaller package a reliable, accurate and relatively inexpensive velocity sensor with an improved linear output range.